bba93a5 set NULL parent window for desktop dialogs, but this change caused
some dialogs to appear on the wrong desktop (eg with openbox). This commit
reverts those changes so desktop dialogs are again parented by the root
window.
WMs that refuse to pop dialogs to the top when parented by the root window
probably need to be fixed.

With a new config, show devices, close, reopen, devices were again hidden.
main-window.c show_panels 'no config exists for this panel and mode - copy'
was run again even after config did exist because panel1_slider_positions0
was not saved in session because old_set "slider_positions" x y and s were
all NULL.